1. Field of the Invention
The present invention relates generally to X-ray tube sources, such as mobile, miniature X-ray tube sources, and more particularly to the geometry of the cathode used in a miniature X-ray tube to reduce unintended electrical field emissions.
2. Related Art
In an X-ray tube, electrons emitted from a cathode source are attracted to an anode by the high bias voltage applied between these two electrodes. The intervening space must be evacuated to avoid electron slowing and scattering, and also to prevent ionization of containment gas and acceleration of the resulting ions to the cathode where they erode the filament and limit tube life. Characteristic and Bremsstrahlung X rays are generated by electron impact on the anode target material. Every material is relatively transparent to its own characteristic radiation, so if the target is thin, there may be strong emission from the surface of the target that is opposite the impacted surface. This arrangement is termed a transmission type X-ray tube.
Miniature transmission type X-ray tubes have been developed that are highly mobile. Current mobile, miniature x-ray sources use a low-power consumption cathode element for mobility, and an anode optic for creating a field free region to prolong the life of the cathode element. These miniature x-ray sources have an electric field that is applied to the anode and cathode which are disposed on opposite sides of an evacuated tube. The anode includes a target material that produces x-rays in response to impact of electrons. The cathode includes a cathode element to produce electrons which are accelerated towards the anode in response to an electric field between the anode and the cathode.
In such miniature x-ray sources the evacuated tube or bulb is an elongated cylinder that is formed of a ceramic material, such as aluminum oxide. The cathode is attached at an end of the tube and the anode is attached at an opposite end of the tube. The cathode is formed of a metal material and is attached by brazing the cathode to the ceramic tube. The joint between the cathode and the tube forms what is known as the triple point interface where the ceramic cylinder, the metal cathode, and the brazing material intersect.
A relatively high electric field is maintained between the cathode and the anode in order to accelerate electrons from the cathode toward the anode. Extremely high electric fields may exist upon certain features of the device, causing electrical arcing between the opposing electrodes. These particularly tend to originate from the interface between metallic cathode components, insulative structure, and vacuum in the device interior. Aside from arcing, the trajectory of the primary electron beam responsible for x-ray generation can be altered due to the presence of unintended stray charge generated at the same metal-dielectric-vacuum interface, often termed the “triple point”.
Current miniature x-ray tube geometry places the triple point in a region subject to high electric field intensity, taking no particularly effective measure to avoid the aforementioned adverse effects. Thus, arcing between the cathode and anode and unintended field emission are likely to occur, compromising the performance and shortening the life of the device as a whole. Electrons from the field flow to be deflected by the triple point interface resulting in a distorted or misdirected electron beam and subsequent x-ray emission pattern.